The development of plastic materials has resulted in the substantial growth of industry directed to the production of useful articles from this versatile medium. Over the years, this production industry has come to be one of the major sectors of manufacturing commerce in the United States as well as in numerous other countries, as well. The usefulness of plastic materials and the growth of this industry has caused a variety of production techniques to be invented and improved. Examples of different production techniques include injection molding, blow molding and thermoforming, to name a few. The focus of the present invention is on the thermoforming of plastic materials.
Thermoforming, as a production method, broadly contemplates the configuration of a plastic sheet into useful articles by heating a plastic production sheet of material into a plastic state, causing the sheet to conform to the shape of a mold and then cooling the molded article. Although different molding processes have been developed, one of the most widespread techniques in thermoforming plastic sheets is that known as vacuum forming. In this technique, a female cavity is configured into the shape of the article to be formed. A passageway communicates with this cavity and is switchably connected between a vacuum source (negative pressure source) and an air pressure source. The sheet of formable plastic material is then placed over the mouth of the female cavity, and the cavity is connected to the negative pressure source so that the sheet of material is sucked into the cavity and adopts the shape of the cavity. Once the part is formed, the passageway is connected to the pressurized air source so that the formed part is ejected from the mold.
In many applications, this thermoforming technique employs a male plug assist which bears against the surface of the plastic material opposite the female cavity and assist in pushing the sheet into the cavity as the vacuum is applied. Plug assists are typically used for deep cavity molding. The plug assist method and apparatus is useful in that it helps control the distribution, i.e., layer thickness, of the production sheet, as the article is formed since the plug assist tends to stretch the plastic materials. Factors that affect the material distribution during stretching include the degree of plasticity of the production sheet, the size and shape of the plug assist, and the construction material and surface finish of the plug. The factors each can affect the slippage of the material along the plug assist surface.
While a variety of different plastic materials have been utilized in the formation of production articles, one of the more commonly used materials is amorphous polyethylene teraphthalate (PET) traditionally extruded as an amorphous sheet to be thermoformed. In its state, PET is readily adaptable to the various thermoforming techniques and is configured into a vast array of commercial articles having high impact resistance.
Recently, however, the industry has recognized certain commercial value in the transformation of amorphous PET into crystallized polyethylene teraphthalate (CPET) during formation of molded articles. The increased crystallinity of this material results in articles that have increased density and which are more resistant to elevated temperatures. However, the transformation of amorphous PET sheets into CPET articles is the only example known to the inventor of a plastic that is drastically changed in terms of its properties during formation. Thus, virtually every aspect of the production of CPET articles departs substantially from that equipment and techniques used in the thermoforming of amorphous PET and other plastic materials.
While CPET technology may find increasing application in many industries, the food packaging industry has immediately been impacted since the value of CPET containers, especially trays, is readily apparent and since the medium has been approved by the appropriate regulatory agencies. The reason for this positive reception is that CPET trays, due to their higher thermo properties, are suitable for use in both microwave and conventional ovens. Thus, these trays are known as "dual-ovenable" and are employed in the packaging of frozen foods. Not only are CPET trays attractive for table use, these trays are lighter and more stain resistant than heavier thermoset polyester alternatives. One producer of PET material projected that over 2 billion CPET trays will be produced by the year 1990.
As noted above, the transformation of amorphous PET into CPET during thermoforming is not without its problems. As with any production thermoforming process, the production rate of the molding articles are a function of two variables: (1) mold cycle time; and (2) the number of articles produced during each machine cycle. For a given mold size, then, the production rate is directly proportional to the mold cycle time. Therefore, with most man-produced items, manufacturing economies are dictated by the production rate of the items. Accordingly, the production cost of CPET articles is largely a function of mold cycle time.
It has been found desirable that amorphous PET be transformed into CPET having a degree of crystallization between 20% and 40%. Crystallization in turn is a function of temperature and time. To accomplish this desired crystallization during a mold cycle, the amorphous PET sheet is usually first preheated to initiate crystallization and is then formed by a heated molding unit; to do otherwise unnecessarily increases the time the article remains in the heated mold. However, if an excessive amount of initial crystallization occurs, the production sheet becomes stiffer. When the sheet is molded into articles, defects in wall thickness can occur in the articles due to the uneven mechanical stretching of the material by the plug assist and by the lack of flowability of the stiffer sheet over the plug assist. Thus, existing apparatus and methods utilize preheaters which raise the initial crystallinity to approximately seven percent (7%) and, at an upper range of less than ten percent (10%).
Two methods have evolved in molding the production sheet that has some initial level of crystallization. In one method, unheated pressure forming air is applied to the side of this sheet opposite the vacuum cavity as the vacuum is applied to assist in forcing the partially crystallized CPET sheet into the female cavity. This occurs as the plug assist is also advancing into the cavity against the surface of the production sheet. Once in the cavity, the sheet must remain until the desired level of crystallization takes place. This method, called the "single step method" loses cycle time because the unheated pressure forming air cools the production sheet and the plug assist. The pressure forming air is at relative low positive pressure, usually less than 10 psi, to avoid excessive cooling. With this lower pressure, the incoming CPET sheet must have a very low initial crystal content, otherwise the sheet is too stiff for the conforming step. As a result of the lower initial crystallization level, the sheet must remain in the mold cavity longer in order to reach the ultimately desired crystallization level. After a suitable time in the heated mold cavity, the mold is then opened and the part ejected. At this point, the part is still at an elevated temperature, and crystallization continues. Furthermore, since the part is still hot and in a plastic state, warping can occur as a result of gravitational forces. Accordingly, in this method it is difficult to control the level of crystallization, and this technique also can result in over-crystallization of the plastic material since it is difficult to control the rate of cooling of the article when it is ejected from the female mold.
A second method of forming CPET articles is referred to as the "two step process" and is basically the same as the single step method, but adds an identical set of second molds for cooling purposes only. This second set of cold molds close on the molded articles to arrest crystallization and to help prevent warping of the part. This method relies on the accurate indexing of the formed and crystallized hot parts into the second station. Vacuum and pressurized air are used to force the molded parts into contact with the cooling mold. This second method has its own problems since any inaccuracies in the indexing will damage the parts as will the uneven shrinking and distortion of the molded parts. Since good contact is difficult to obtain uniformly over the part, those parts in contact with the cool mold will exhibit arrested crystallization while those areas not making good contact will continue to crystallize to a higher level. Thus, there is a lack of uniformity of crystallization over the volume of the part. This technique is, naturally, more expensive, since it requires an additional second set of cooling molds. Further, the problem in indexing a two step method gets more critical as the size and number of mold cavities increases.
In both the one step method and the two step method, as heretofore practiced, the cycle time of the molding apparatus is retarded. In the one step process, the use of low pressure forming air sometimes results in less intimate contact of the production sheet with the heated female cavity so that there is lower thermal transfer thus slowing the rate of crystallization. Further, contact between the preheated production sheet and the cooler plug assist during the closing of the mold also tends to retard crystallization. This is also true because the pressure forming air is unheated so that flow of the unheated pressure forming air tends to cool both the sheet and the plugs.
In the second method, the crystallization is entirely arrested by the second set of molds. Thus, almost the entire crystallization in the forming process must occur while the part is in the first molded cavity. Thus, the production sheet must remain in the cavity of the first mold for a longer time.
In either case, typical mold cycle times may be as long as 12 seconds. Recent improvements in materials technology accomplished by the addition of nucleating agents into the resin materials used to form the amorphous production sheet, have reduced cycle times to approximately 6 seconds since the nucleating agents enhance the rate of crystallization. However, the use of nucleating agents to increase crystallization rates necessarily increases the expense of the polyester resin used to produce the production sheet. Also, the use of nucleating agents require great control of the preheating of the production sheet to initiate crystallization as the sheet is to be indexed into the mold.